flags: use common dumpxml flags check

flags: use common dumpxml flags check

The previous patches only cleaned up ATTRIBUTE_UNUSED flags cases;
auditing the drivers found other places where flags was being used
but not validated.  In particular, domainGetXMLDesc had issues with
clients accepting a different set of flags than the common
virDomainDefFormat helper function.

* src/conf/domain_conf.c (virDomainDefFormat): Add common flag check.
* src/uml/uml_driver.c (umlDomainAttachDeviceFlags)
(umlDomainDetachDeviceFlags): Reject unknown
flags.
* src/vbox/vbox_tmpl.c (vboxDomainGetXMLDesc)
(vboxDomainAttachDeviceFlags)
(vboxDomainDetachDeviceFlags): Likewise.
* src/qemu/qemu_driver.c (qemudDomainMemoryPeek): Likewise.
(qemuDomainGetXMLDesc): Document common flag handling.
* src/libxl/libxl_driver.c (libxlDomainGetXMLDesc): Likewise.
* src/lxc/lxc_driver.c (lxcDomainGetXMLDesc): Likewise.
* src/openvz/openvz_driver.c (openvzDomainGetXMLDesc): Likewise.
* src/phyp/phyp_driver.c (phypDomainGetXMLDesc): Likewise.
* src/test/test_driver.c (testDomainGetXMLDesc): Likewise.
* src/vmware/vmware_driver.c (vmwareDomainGetXMLDesc): Likewise.
* src/xenapi/xenapi_driver.c (xenapiDomainGetXMLDesc): Likewise.